As is known in the art, compact discs (CDs) are formed with a central opening and with a boss surrounding the opening on one side of the disc. Normally, in the course of manufacture of the discs, a number of discs are placed on a spindle with the bosses forming spaces between adjacent discs outside the bosses. Obviously, it is desirable to know the number of discs carried by the spindle.
Higgison et al U.S. Pat. No. 4,994,666 shows a device for counting compound discs stacked on a spindle by means of a laser beam which traverses the spaces between adjacent discs of the stack. More specifically, a base supports a laser, the beam of which is translated to a lens system supported on a platform. The platform is driven by a lead screw to cause the beam to scan a stack of discs. A detector supported by the platform and disposed on the opposite side of the stack from the transmitting optics produces a signal each time the laser beam traverses the space between a pair of adjacent discs, thus to produce a count of the number of discs in a stack.
While the Higgison et al device is generally satisfactory, it embodies a number of defects. First, owing to the fact that it employs movable optics, the system is relatively complex and cumbersome. Secondly, it involves the possibility of missing a count because a pair of adjacent discs have no space therebetween. This may result from production defects or from the fact that not all discs are stacked with their bosses facing the same way.